There are a large number of products in industry that are encased in plastic film packets. Many of these products require the presence of a vacuum within the packets, such as for the preservation of food and drug products. Another important sealed product is the super-insulator packets of precipitated silicon dioxide powder and similar powders having low thermal conductivity in vacuum. In these and other applications it is desirable to exclude air and other gases to prevent various types of reactions with the material that is enclosed and to prevent undesirable effects on the physical properties of the packets, e.g. thermal conductivity.
There has been a need, therefore, to measure the value of the internal vacuum after the sealing operation, as well as after storage for any length of time. This measurement needs to be non-intrusive, i.e., there should be no potential intrusion into the packet.
One instrument that has been utilized in the prior art involves immersing the entire sealed packet into a vacuum system, and then "interrogating" the packet optically for movement of the film envelope when the chamber vacuum falls below the packet vacuum. This type of equipment is expensive and, more important, is time consuming when each packet must be individually tested in the vacuum chamber. In particular, this type of equipment is not readily amenable to inclusion in production line testing.
References that may be pertinent to the evaluation of this invention are U.S. Pat. Nos.: 4,478,070 issued to E. W. Clifford, et al, on Oct. 23, 1984; 4,715,215 issued to J. M. Perhach, et al, on Dec. 29, 1987; and 4,747,299 issued to J. M. Fox, et al, on May 31, 1988. All of these devices require extensive (and thus expensive) equipment, and since each packet must be evaluated, the testing is time consuming with these devices of the prior art.
Accordingly, it is an object of the present invention to provide a simple instrument that quickly measures the vacuum within sealed thin-walled packets.
It is another object of the present invention to provide a simple instrument that rapidly measures the vacuum within sealed plastic film packets.
Another object of the present invention is to provide an instrument wherein an external vacuum is applied to a localized area on the exterior of sealed plastic film packets, with means to determine when the film lifts from the content of the packet under the influence of that vacuum applied in the localized area.
Still another object of the present invention is to provide an instrument wherein an external vacuum is applied to a localized area on one exterior surface of sealed plastic film packets, with the opposite exterior surface of the packet being maintained undisturbed by that applied vacuum, with means to determine when the film lifts from the content of the packet under the influence of that vacuum applied in the localized area.
It is also an object of the present invention to provide a hand-held instrument, or one that can be used on a production line, to measure the vacuum within sealed plastic film packets.
These and other objects of the present invention will become apparent upon a consideration of the following detailed description when taken together with the drawings referenced hereinafter.